The present application relates generally to sonar systems, and more specifically to systems and methods of increasing the accuracy of sonar range measurements.
Sonar systems are known that employ sonar pulses reflected from an object or target to estimate a distance to the target (also known as estimating the range of the target). Such sonar systems include side-scan sonar systems, forward and downward-looking sonar systems, 2-D and 3-D sonar systems, and synthetic aperture sonar systems. A conventional sonar system for performing sonar range estimation is typically configured to transmit one or more sonar pulses comprising sonic or supersonic pressure waves toward a selected target, and to receive one or more sonar pulses reflected from the target. Such reflected sonar pulses, which are commonly called echoes or returns, may include a significant amount of background noise and/or other interfering signals in addition to a reflected sonar signal of interest. The conventional sonar system typically includes a coherent receiver (also known as a matched filter receiver) configured to: receive both the echo and a representation of the transmitted sonar pulse. For example, the coherent receiver may comprise a cross correlator. The echo and the representation of the transmitted sonar pulse are cross-correlated within the coherent receiver to generate a peak cross correlation value, which is compared to a predetermined threshold value. If the cross correlation value is greater than the predetermined threshold value, then the reflected sonar signal of interest has been successfully detected. The conventional sonar system then utilizes the cross correlation peak to obtain a measure of the range of the target.
One drawback of the above-described conventional sonar system is that the level of background noise and/or other interfering signals contained within the echo or return may be sufficient to cause the reflected sonar signal to go undetected or to be falsely detected, thereby causing the cross correlator to produce inaccurate range measurements. Such inaccurate range measurements are likely to occur in low signal-to-noise ratio (SNR) sonar environments, in which the noise power within the echo may be comparable to or greater than the reflected signal power. This can be problematic in sonar range estimation systems because a reduction in the measurement accuracy of the cross correlator typically leads to a concomitant reduction in sonar range accuracy.
Prior attempts to increase the accuracy of sonar range measurements in noisy sonar environments have included filtering out at least some of the background noise before providing the echo to the cross correlator. However, such attempts have generally not worked well enough to allow successful detection of reflected sonar signals and accurate estimation of range in low SNR sonar environments. This is due, at least in part, to the fact that sonar systems typically receive sonar pulses that include various types of noise from a variety of different noise sources. For example, a sonar system may transmit sonar pulses through a medium such as water from a ship or submarine that produces noise across a wide range of frequency. Further, other ships, submarines, or structures producing noise across wide frequency ranges may be within the vicinity of the sonar system. Moreover, the natural interaction of the water and objects within the water including the selected target may produce a substantial amount of ambient noise.
Various adaptive beam forming and noise cancellation techniques have been employed in an attempt to minimize the effect of noise on sonar range estimation. However, such techniques generally require some knowledge of the characteristics of the noise source. For this reason, such techniques are usually most effective when used to minimize the effect of the noise produced by the ship or submarine carrying the sonar system, which is relatively easy to characterize. However, such techniques are typically ineffective when the source of the noise is unknown.
In addition, sonar ranging systems may receive echoes from a plurality of selected (and unselected) targets, each target having its own associated noise level, and it may be desirable to determine the noise level and range of each target separately. Such noise associated with multiple targets may be stationary or non-stationary, linear or nonlinear, or additive or non-additive. Further, at least some of the background noise may result from reverberations and/or random signal distortions of the transmitted or reflected sonar pulse, and therefore both the noise level and its structure may be significantly affected by the transmitted sonar signal. However, conventional sonar systems are generally incapable of accurately estimating noise levels and target ranges in the presence of non-stationary, nonlinear, non-additive, and/or signal-dependent noise.
Moreover, the density and temperature of the transmission medium (e.g., water) and the frequency of the transmitted/reflected sonar signal may affect the decay rate of the sonar pulse propagating through the medium. In addition, the absorption of certain frequencies of the transmitted sonar pulse by the target may affect the strength of the resulting echo or return. The sonar pulse strength, frequency range, medium dispersion properties, shape, and duration may also affect the accuracy of sonar range measurements. However, conventional sonar systems are generally incapable of fully compensating for such factors when called upon to generate accurate noise and range estimates.
It would therefore be desirable to have a system and method of performing sonar range estimation that takes into account the effects of background noise whether the noise is stationary or non-stationary, linear or non-linear, additive or non-additive, or signal-dependent or non-signal-dependent. It would also be desirable to have a method of performing sonar range estimation that can be used to increase sonar ranging accuracy in low SNR sonar environments.